Ceramic glazes have normally been prepared by ball-milling together a glass frit, a clay and an inorganic pigment along with other components typically found in a raw glaze mix followed by dispersing the ball-milled product in water and spraying the dispersion on to a refractory substrate. The substrate is then fired to a temperature sufficient to melt the raw glaze. The molten glaze flows over and iniformly covers the surface of the substrate, totally surrounding and fusing the pigment to the surface. Upon cooling, the surface is covered with a hard glaze that is durable, washable and chemically inert in addition to being attractive in appearance.
This prior art procedure of ball-milling a frit and pigment has several serious drawbacks. The color intensity of inorganic pigments is dependent at least partially on the particle size of the pigments. For most high temperature pigments used to color ceramic stains, the color value is strongest when the particle size is in the 3 to 4 mu (microns) range or possibly larger. Typically, however, when the pigment is milled with the glaze composition, the pigment passes through the optimum range, eventually reaching a particle size of 2 mu or smaller. At this size, the color intensity is noticeably weaker than it is in the 3-4 mu range due to attack by the molten glaze during firing.
At intervals, while ball-milling, the glaze is tested for color and shade, and adjustments are made in the composition, often resulting in the addition of more of the same pigment, or the use of an entirely different pigment. Each addition requires several additional hours on the ball mill and results in potentially even greater particle size reduction.
When the pigmented glaze is removed from the ball mill, the mill must be thoroughly cleaned before a color change to prevent color contamination from one batch to the next.
If the manufacturer attempts to minimize the amount of particle size reduction of the pigment by adding the pigment near the end of the ball-milling, the resultant glaze often appears mottled due to inadequate wetting of the pigment by the aqueous raw glaze.
U.S. Pat. No. 2,057,958 describes one method of preparing a vitreous enamel slip using a titanium dioxide pigment. The pigment is prepared as a floc or a gel and is washed to remove soluble salts. It is added as an aqueous slurry to a ball mill along with a glass frit, a small amount of an adhesive such as gum tragacanth and from 0 to 3% clay. If a colored enamel is desired, a coloring agent is added to the mill. The basic problems discussed previously, namely the ball-milling of the pigment with the lack of control over the color intensity is manifest because of the intensive ball-milling.
U.S. Pat. No. 2,607,701 relates to glass colors adapted to be applied to a substrate by stencilling. The colors are composed of an inorganic pigment and a glass frit in a vehicle composed of 20 to 98% of a wax having a melting point between 120.degree. F. and 212.degree. F., 2 to 15% of ethyl cellulose as a viscosity pormoter and hardening agent, 0 to 55% of a resin or its derivative useful as a hardener, and a binder to dissolve the wax. Lecithin or phosphorated tall oil is present in an amount of 0 to 20% as a dispersion control agent. The vehicle is used in an amount of 1 part to 1-4 parts of the glass color. After the glass color is applied to the substrate, the glass is fused by firing.
U.S. Pat. No. 3,443,973 describes a method of producing vitreous enamel powders by preparing an aqueous slurry of a borosilicate frit having a pH of at least 8, preparing an aqueous slurry of a refractory inorganic pigment having a pH of at least 8, preparing an aqueous slurry of a refractory inorganic pigment having a pH no higher than 5, mixing the two slurries to provide from 3 to 20 parts of pigment per 100 parts of frit to cause agglomeration of the frit and pigment particles, drying and heating the particles to form a loosely bonded cake and breaking up the cake to form individual frit-pigment particles.
U.S. Pat. No. 3,868,334 discloses a glass composition for firing into a resistant glaze and comprising an admixture of a borosilicate glass frit, a conductive metal oxide such a ruthenium dioxide, and a binder composed of ethyl cellulose in butyl carbitol.
U.S. Pat. No. 4,331,768 describes a method of forming a smooth, matte glaze on a ceramic substrate comprising forming a base frit and preparing a slurry of the frit plus 4-8% of SnO.sub.2 and 4-8% of TiO.sub.2, said frit optimally containing up to 5% of an organic binder such as sugar and up to 5% of clay. The slurry is then applied to the substrate whereupon the substrate is fired at a temperature of 975.degree.-1075.degree. C.